An electrochemical process is a physical process enabling the conversion between electrical energy and chemical energy. Various applied electrochemical processes have been studied as clean and high efficiency industrial process or power generating system. These processes have been used on an industrial basis.
For these electrochemical processes, a liquid electrolyte having an ionically-conductive supporting electrolyte dissolved therein has been used. In recent years, the development of an electrochemical device utilizing a solid electrolyte has been under way. Unlike the liquid electrolyte, such a polymer solid electrolyte is advantageous in that it allows easy separation of a water-soluble electrolysis product and enhancement of the purity of gaseous product, needs only to control the supply of water in the case of electrolysis of water, eliminating the necessity of controlling the electrolyte concentration, and allows the use of a safe and simple electrolytic cell structure.
For example, in the electrolysis of salt, which is employed on a large scale in the art of industrial electrolysis, mercury process or asbestos process, which had heretofore been used, has been replaced by a process using an ion exchange membrane made of a fluororesin, which is a polymer solid electrolyte.
Further, in the art of lithium battery, a polymer solid electrolyte having a fluorine-containing electrolyte retained on a resin as a matrix has been developed. Moreover, in the art of a fuel cell using hydrogen and oxygen as starting material, the public interest has been switched from the alkaline solution type or phosphoric acid type cell, which is a conventional type, to a polymer solid electrolyte type fuel cell (PEFC). As compared with the conventional types of cell, this polymer solid electrolyte type fuel cell is simple and can provide a large output density and thus has become an important target of development as a clean power supply for automobile, local power generator, etc.
These electrochemical devices often comprise as the material of polymer solid electrolyte a resin composed of a fluororesin skeleton having a sulfonic acid group as an exchange group, though being expensive, to have a high durability which could not be obtained with hydrocarbon-based resins.
The polymer solid electrolyte to be used in the above-described cells and electrolytic cells must meet essential requirements for high ionic conductivity and capability of separating reactant and product as well as low electrical resistivity from the standpoint of energy efficiency. In particular, in the case these devices are operated at a high current density, the electrical resistivity is an important factor that may disable the electrochemical process if it is too high.
In order to reduce voltage loss due to the electrical resistivity of the polymer solid electrolyte, it has been practiced to reduce the thickness of the polymer solid electrolyte formed in a sheet. The thickness of a perfluorocarbon sulfonic acid-based polymer solid electrolyte membrane, which has heretofore been about 200 μm, is now reduced to a range of from about 25 to 50 μm.
By thus reducing the thickness of the polymer solid electrolyte sheet, the loss due to resistivity can be reduced. On the other hand, however, the physical strength of the polymer solid electrolyte membrane itself is reduced, making it difficult for the polymer solid electrolyte membrane to handle. In particular, the polymer solid electrolyte type fuel cell, which has recently been noted, is best required to produce a raised power voltage for the purpose of raising the energy efficiency. An ordinary membrane-electrode-membrane electrode (MEA: Membrane Electrode Assembly) is required to have a strength high enough to form an electrode on the surface of the polymer solid electrolyte. To this end, the strength of the membrane is secured at the sacrifice of electrical resistivity. As previously mentioned, a low electrical resistivity polymer solid electrolyte membrane having a thickness of 25 μm has been developed. Nevertheless, it is the status of quo that the thickness of the membrane is raised, although being aware of the increase of electrical resistivity, and reinforced by a reinforcing material to enhance the mechanical strength thereof or a polymer solid electrolyte membrane (ion exchange membrane) reinforced by a reinforcing material is originally produced. A material having a sufficiently small thickness and a high strength has been developed as a reinforcing material and can be used for polymer solid electrolyte membrane. However, since the reinforced part of the membrane does not pass electric current, the resulting membrane is still unavoidably subject to the increase of loss due to resistivity.
The reduction of the electrical resistivity of the membrane can be attained by the increase of the ion exchange capacity of the membrane besides by the reduction of the thickness of the membrane. However, when the ion exchange capacity of the membrane is raised, the resulting membrane normally exhibits deteriorated film-forming properties, making it impossible to obtain a sufficient membrane strength. Accordingly, the ion exchange capacity of the membrane cannot be raised excessively.
For these reasons, the practical use of polymer solid electrolyte membrane has been prevented by the deterioration of mechanical strength although a polymer solid electrolyte membrane having a sufficiently low electrical resistivity has been available.
On the other hand, when the connection between the membrane and the electrode is insufficient, local concentration of electric current can easily occur, making it difficult to obtain a good cell performance. In order to overcome these difficulties, Japanese Patent Laid-Open No. 1994-251782 and Japanese Patent Laid-Open No. 1996-329962 disclose a process for the preparation of an assembly of a perfluorocation exchange membrane and a fuel cell electrode and an assembly of an electrode having an electrode catalyst and an electrolyte incorporated in a porous PTEFE and an electrolyte, respectively. Japanese Patent Laid-Open No. 1998-284087, Japanese Patent Laid-Open No. 1998-334923 and Japanese Patent Laid-Open No. 1999-288727 disclose a process which comprises obtaining an assembly of electrode catalyst layers containing a proton-conductive polymer having different ion exchange capacities to provide a fuel cell having an enhanced performance Further, Japanese Patent Laid-Open No.1999-135136 and Japanese Patent Laid-Open No. 1999-339824 disclose a process for the preparation of an assembly of an ion exchange membrane having a phosphonic acid group and an electrode. Moreover, WO 00/63991 discloses a process for the synthesis of a novel ion exchange membrane having a phosphonic acid group and hence a reduced resistivity.
These improvements provide a great enhancement of cell performance and electrolytic properties. However, these improvements are disadvantageous in that the resulting assembly has no sufficient strength and the membrane components employable are limited, thus providing only a polymer solid electrolyte membrane having a low resistivity but deteriorated film-forming properties.